The present invention relates to an electromagnetic proportional control valve apparatus which is operated by the utilization of pilot pressure.
A conventional electromagnetic proportional control valve apparatus of the kind referred to above comprises a valve body as disclosed in Japanese Patent Application Laid-Open (Provisional Publication) No. Sho 62-261782. The valve body is provided therein with a guide bore, and a supply port, a control port and a discharge port which are formed in a peripheral wall surrounding the guide bore. A spool is accommodated in the guide bore for sliding movement therein. A valve seat element is fixedly mounted to one end of the guide bore. A pilot chamber is defined between opposed end faces of the respective valve seat element and spool. The spool is abutted against the valve seat element under biasing force of a first return spring, in an unexcited state of electromagnetic drive means to be described later.
The end face of the spool, which is opposed to the valve seat element, is formed with an axial bore extending along an axis of the spool. A first restricting bore is formed in a peripheral wall surrounding the axial bore. The pilot chamber communicates with the supply port through the axial bore and the first restricting bore.
The valve seat element is formed therein with a valve bore, and a second restricting bore through which one end of the valve bore communicates with the pilot chamber. The other end of the valve bore has a peripheral edge which serves as a valve seat. A pilot valve is so arranged as to face toward the valve seat. During deenergization of the electromagnetic drive means to be described later, the pilot valve is spaced away from the valve seat under biasing force of a second return spring.
Electromagnetic drive means is arranged at an end of the valve body on the side of the valve seat element. When electric current is caused to pass through the electromagnetic drive means, force substantially in proportion to the current is given to the pilot valve, so that the pilot valve moves toward the valve seat against the biasing force of the second return spring. As a result, pilot pressure substantially in proportion to the current supplied to the electromagnetic drive means is produced in the pilot chamber due to pressure drop which occurs in an annular passage defined between the pilot valve and the valve seat, and due to pressure drop which occurs in the first and second restricting bores. By the pilot pressure, the spool is moved away from the valve seat element against the biasing force of the first return spring.
The spool has an outer periphery thereof which is formed with a land section for controlling communication between the control port and the supply port, and a land section for controlling communication between the control port and the discharge port. The spool moves such that force obtained by multiplication of the control-port pressure by a difference in pressure receiving area between both the lands is balanced with force due to the pilot pressure, thereby controlling communication among the ports. Thus, pressure at the control port is so controlled as to be substantially in proportion to the current supplied to the electromagnetic drive means.
In the electromagnetic proportional control valve apparatus constructed as above, the second restricting bore in the valve seat element bears such a role as to cause pressure drop occurring at the second restricting bore to bring pressure within the valve bore to a value lower than the pilot pressure, thereby enabling relatively low exciting force to control the relatively high pilot pressure. Further, the second restricting bore bears also such a role as to produce a damper effect on the spool. Specifically, when the spool moves, working fluid within the pilot chamber is caused to pass through the second restricting bore, and the moving speed of the spool is restrained low due to flow resistance which occurs at passage of the working fluid through the second restricting bore.
Assuming that there is no damper effect due to the second restricting bore, then there arises the following disadvantage. That is, the spool moves in response to fluctuation in the pilot pressure attendant upon fluctuation in the electric current supplied to the electromagnetic drive means, or in response to fluctuation in the control-port pressure, to control communication between the control port and the supply port and/or the tank port. Because of inertia of the spool at its movement, however, the spool passes largely over an optimum position, so that the balance between the control-port pressure and the pilot pressure is broken. Accordingly, the spool moves in the reverse direction. Also at the movement in the reverse direction, the spool passes largely over the optimum position because of inertia of the spool. Such repetition of reciprocative movement of the spool, that is, hunting makes the control unstable. Further, when the current supply to the electromagnetic drive means is halted, the pilot valve is moved away from the valve seat so that the pilot pressure is reduced. Following the reduction in the pilot pressure, the spool is returned toward the valve seat element from a position where the spool controls the pressure at the control port. At this time, the spool impinges against the valve seat element at high speed.
In the control valve apparatus disclosed in the aforesaid Japanese patent, the damper effect on the spool occurring due to the second restricting bore formed in the valve seat prevents the spool from impinging against the valve seat element at high speed, and prevents the spool from hunting.
Particularly, high damper effect is required when the pressure at the control port is high. This damper effect can be enhanced by reducing the opening area of the second restricting bore to increase the flow resistance of the working fluid. Further, since the viscosity resistance of the working fluid is reduced when the control valve apparatus is used under high-temperature environment, it is desired to further reduce the opening area of the second restricting bore.
However, the opening area of the second restricting bore in the valve seat element cannot be reduced freely for the reason discussed below. That is, if the opening area of the second restricting bore is reduced, the flow resistance at the second restricting bore makes it difficult that the pilot pressure within the pilot chamber escapes to the atmosphere through the second restricting bore, when the current supplied to the electromagnetic drive means is reduced or is brought to zero to thereby move the pilot valve away from the valve seat. Thus, it takes a considerable time to lower the pressure at the control port in response to lowering or halt of the supply current.
It has been difficult to determine the opening area or the flow resistance of the second restricting bore so as to sufficiently satisfy both the damper effect and the response.
Additionally, U.S. Pat. No. 4,763,872 discloses an electromagnetic proportional control valve apparatus which has no second restricting bore.